Why did the Arab invasions of the medeival times begin?

Answer 1

For centuries the Arabs were divided into many tribes, each saw itself as nation in its own right. With the rise of Islam came the rise of the Arabs, it was the prophet Mohammed who united the Arab tribes and the powerful since of tribalism ended. The Arabs were motivated to create an empire and spread God's message to humanity. Thus they began their conquests which made them end as far as Southern France in the West and in the East to the borders of China. The Arabs who were always dominated by other people, now became the dominant peoples.